This invention relates to a method and circuit for an accurate voltage reference, and more specifically to a differential circuit that uses a feedback loop and dual conduction of tunnel devices to accurately program a desired charge level on a floating gate.
Programmable analog floating gate circuits have been used since the early 1980""s in applications that only require moderate absolute voltage accuracy over time, e.g., an absolute voltage accuracy of 100-200 mV over time. Such devices are conventionally used to provide long-term non-volatile storage of charge on a floating gate. A floating gate is an island of conductive material that is electrically isolated from a substrate but capacitively coupled to the substrate or to other conductive layers. Typically, a floating gate forms the gate of an MOS transistor that is used to read the level of charge on the floating gate without causing any leakage of charge therefrom.
Various means are known in the art for introducing charge onto a floating gate and for removing the charge from the floating gate. Once the floating gate has been programmed at a particular charge level, it remains at that level essentially permanently, because the floating gate is surrounded by an insulating material which acts as a barrier to discharging of the floating gate. Charge is typically coupled to the floating gate using hot electron injection or electron tunneling. Charge is typically removed from the floating gate by exposure to radiation (UV light, x-rays), avalanched injection, or Fowler-Nordheim electron tunneling. The use of electrons emitted from a cold conductor was first described in an article entitled Electron Emission in Intense Electric Fields by R. H. Fowler and Dr. L. Nordheim, Royal Soc. Proc., A, Vol. 119 (1928). Use of this phenomenon in electron tunneling through an oxide layer is described in an article entitled Fowler-Nordheim Tunneling into Thermally Grown SiO2 by M. Lenzlinger and E. H. Snow, Journal of Applied Physics, Vol. 40, No. 1 (January, 1969), both of which are incorporated herein by reference. Such analog floating gate circuits have been used, for instance, in digital nonvolatile memory devices and in analog nonvolatile circuits including voltage reference, Vcc sense, and power-on reset circuits.
FIG. 1A is a schematic diagram that illustrates one embodiment of an analog nonvolatile floating gate circuit implemented using two polysilicon layers formed on a substrate and two electron tunneling regions. FIG. 1A illustrates a cross-sectional view of an exemplary prior art programmable voltage reference circuit 70 formed on a substrate 71. Reference circuit 70 comprises a Program electrode formed from a first polysilicon layer (poly1), an Erase electrode formed from a second polysilicon layer (poly2), and an electrically isolated floating gate comprised of a poly1 layer and a poly2 layer connected together at a corner contact 76. Typically, polysilicon layers 1 and 2 are separated from each other by a thick oxide dielectric, with the floating gate fg being completely surrounded by dielectric. The floating gate fg is also the gate of an NMOS transistor shown at 73, with a drain D and a source S that are heavily doped n+ regions in substrate 70, which is P type. (The number zero is also referred to as xe2x80x9c0xe2x80x9d or Ø herein.) The portion of dielectric between the poly1 Program electrode and the floating gate fg, as shown at 74, is a program tunnel region (or xe2x80x9ctunnel devicexe2x80x9d) TP, and the portion of dielectric between the poly1 floating gate fg and the poly2 erase electrode, shown at 75, is an erase tunnel region TE. Both tunnel regions have a given capacitance. Since these tunnel regions 74,75 are typically formed in thick oxide dielectric, they are generally referred to as xe2x80x9cthick oxide tunneling devicesxe2x80x9d or xe2x80x9cenhanced emission tunneling devices.xe2x80x9d Such thick oxide tunneling devices enable the floating gate to retain accurate analog voltages in the +/xe2x88x924 volt range for many years. This relatively high analog voltage retention is made possible by the fact that the electric field in most of the thick dielectric in tunnel regions 74,75 remains very low, even when several volts are applied across the tunnel device. This low field and thick oxide provides a high barrier to charge loss until the field is high enough to cause Fowler-Nordheim tunneling to occur. Finally, reference circuit 70 includes a steering capacitor CC that is the capacitance between floating gate fg and a different n+ region formed in the substrate that is connected to a Cap electrode.
FIG. 1B is a schematic diagram that illustrates a second embodiment of a floating gate circuit 70 that is implemented using three polysilicon layers. The three polysilicon floating gate circuit 70xe2x80x2 is similar to the two polysilicon embodiment except that, for example Erase electrode is formed from a third polysilicon layer (poly 3). In addition, the floating gate fg is formed entirely from a poly2 layer. Thus, in this embodiment there is no need for a corner contact to be formed between the poly1 layer portion and the poly2 layer portion of floating gate fg, which is required for the two polysilicon layer cell shown in FIG. 1A.
Referring to FIG. 2, shown at 25 is an equivalent circuit diagram for the voltage reference circuit 70 of FIG. 1A and 70xe2x80x2 of FIG. 1B. For simplicity, each circuit element of FIG. 2 is identically labeled with its corresponding element in FIGS. 1A and 1B.
Setting reference circuit 70 to a specific voltage level is accomplished using two separate operations. Referring again to FIG. 1A, the floating gate fg is first programmed or xe2x80x9cresetxe2x80x9d to an off condition. The floating gate fg is then erased or xe2x80x9csetxe2x80x9d to a specific voltage level. Floating gate fg is reset by programming it to a net negative voltage, which turns off transistor TØ. This programming is done by holding the Program electrode low and ramping the n+ bottom plate of the relatively large steering capacitor CC to 15 to 20V via the Cap electrode. Steering capacitor CC couples the floating gate fg high, which causes electrons to tunnel through the thick oxide at 74 from the poly1 Program electrode to the floating gate fg. This results in a net negative charge on floating gate fg. When the bottom plate of steering capacitor CC is returned to ground, this couples floating gate fg negative, i.e., below ground, which turns off the NMOS transistor TØ.
To set reference circuit 70 to a specific voltage level, the n+ bottom plate of steering capacitor CC, the Cap electrode, is held at ground while the Erase electrode is ramped to a high voltage, i.e., 12 to 20V. Tunneling of electrons from floating gate fg to the poly2 Erase electrode through the thick oxide at 75 begins when the voltage across tunnel device TE reaches a certain voltage, which is typically approximately 11V. This tunneling of electrons from the fg through tunnel device TE increases the voltage of floating gate fg. The voltage on floating gate fg then xe2x80x9cfollowsxe2x80x9d the voltage ramp coupled to the poly2 Erase electrode, but at a voltage level offset by about 11V below the voltage on the Erase electrode. When the voltage on floating gate fg reaches the desired set level, the voltage ramp on poly2 Erase electrode is stopped and then pulled back down to ground. This leaves the voltage on floating gate fg set at approximately the desired voltage level.
As indicated above, reference circuit 70 meets the requirements for voltage reference applications where approximately 200 mV accuracy is sufficient. The accuracy of circuit 70 is limited for two reasons. First, the potential on floating gate fg shifts down about 100 mV to 200 mV after it is set due to the capacitance of erase tunnel device TE which couples floating gate fg down when the poly2 Erase electrode is pulled down from a high voltage to ØV. The amount of this change depends on the ratio of the capacitance of erase tunnel device TE to the rest of the capacitance of floating gate fg (mostly due to steering capacitor CC), as well as the magnitude of the change in voltage on the poly2 Erase electrode. This voltage xe2x80x9coffsetxe2x80x9d is well defined and predictable, but always occurs in such prior art voltage reference circuits because the capacitance of erase tunnel device TE cannot be zero. Second, the accuracy of circuit 70 is also limited because the potential of floating gate fg changes another 100 mV to 200 mV over time after it is set due to various factors, including detrapping of the tunnel devices and dielectric relaxation of all the floating gate fg capacitors.
An analog voltage reference storage device that uses a floating gate is described in U.S. Pat. No. 5,166,562 and teaches the uses of hot electron injection for injecting electrons onto the floating gate and electron tunneling for removing electrons from the floating gate. The floating gate is programmed by controlling the current of the hot electron injected electrons after an erase step has set the floating gate to an initial voltage. See also U.S. Pat. No. 4,953,928. Although this method of programming the charge on a floating gate is more accurate than earlier analog voltage reference circuits including a floating gate, the level of accuracy is still on the order of 50 mV to 200 mV.
Prior art floating gate storage devices have sometimes used dual conduction of Fowler-Nordheim tunnel devices, i.e., wherein both the program and erase tunnel elements in a floating gate device are caused to conduct simultaneously in order to provide the coupling of charge onto the floating gate. However, this method has only been used in digital circuits to program the floating gate to either a xe2x80x9c1xe2x80x9d condition or a xe2x80x9c0xe2x80x9d condition to provide memory storage. The precise charge on the floating gate in such applications is not of concern and so is not precisely controlled in such circuits. According to the prior art, such dual conduction digital programming of a floating gate is considered to be a less efficient and desirable way than generating electron conduction through a single tunnel element to control the level of charge on a floating gate. Known disadvantages of dual conduction digital programming of a floating gate include the fact that a larger total voltage is required to provide dual conduction and tunnel oxide trap-up is faster because more tunnel current is required.
An example of a prior art analog nonvolatile floating gate circuit that uses dual conduction of electrons for adding and removing electrons from a floating gate is disclosed in U.S. Pat. No. 5,059,920, wherein the floating gate provides an adaptable offset voltage input for a CMOS amplifier. In this device, however, only one Fowler-Nordheim tunnel device is used. The electrons are injected onto the floating gate using hot electron injection, while Fowler-Nordheim electron tunneling is used to remove electrons from the floating gate, so as to accurately control the charge on the floating gate. This means of injecting electrons onto the floating gate is used because the charge transfer is a controlled function of the voltage on the floating gate. Another example of a prior art dual conduction floating gate circuit is disclosed in U.S. Pat. No. 5,986,927. A key problem with such prior art devices is that they do not compensate for common-mode voltage and current offsets, common-mode temperature effects, and mechanical and thermal stress effects in the integrated circuit.
Applications that require increased absolute voltage accuracy generally use a bandgap voltage reference. A bandgap voltage reference typically provides approximately 25 mV absolute accuracy over time and temperature, but can be configured, to provide increased accuracy by laser trimming or E2 digital trimming at test. While a bandgap voltage reference provides greater accuracy and increased stability over the prior art voltage reference circuits discussed above, a bandgap voltage reference only provides a fixed voltage of about 1.2V. Therefore, additional circuitry, such as an amplifier with fixed gain, is needed to provide other reference voltage levels. Moreover, prior art bandgap voltage references typically draw a relatively significant current, i.e., greater than 10 xcexcA.
What is needed is an analog programmable voltage reference circuit that can be quickly and accurately set to any analog voltage without the need for a fixed-gain amplifier and that provides improved stability and accuracy over time and temperature as compared to prior art voltage references. It is also desirable that the improved stability and accuracy be obtained in a voltage reference circuit that draws significantly less current than prior art voltage references.
The present invention is directed at addressing the above-mentioned shortcomings, disadvantages, and problems of the prior art. The present invention comprises a floating gate circuit including a floating gate having a voltage thereon, a first tunnel device formed between said floating gate and a first tunnel electrode, and a second tunnel device formed between said floating gate and a second tunnel electrode for causing electrons to tunnel onto and off of said floating gate for modifying the voltage on said floating gate as a function of a voltage differential between said first and second tunnel electrodes, a first circuit coupled to said floating gate for causing the voltage on said floating gate to be compared to a first voltage, and a feedback circuit coupled between said floating gate and said first circuit for controlling said voltage differential such that the voltage on said floating gate is modified until said floating gate circuit reaches a steady state condition wherein the voltage on said floating gate is a predetermined function of said first voltage.
The present invention also comprises a differential floating gate circuit, comprising: a) a floating gate; b) a first circuit coupled to said floating gate, said first circuit comprising a first tunnel device formed between said floating gate and a first tunnel electrode and a second tunnel device formed between said floating gate and a second tunnel electrode; c) a second circuit operatively coupled to said floating gate and an input voltage terminal and having an output terminal; and d) a feedback loop coupled between said output terminal and said first tunnel electrode, wherein during a set mode: said first circuit for causing said first and second tunnel devices to operate in a dual conduction mode, under the control of a voltage differential between the first and second tunnel electrodes, for modifying the charge level on said floating gate; said second circuit for comparing the voltage on said floating gate with an input set voltage at said input voltage terminal and generating an output voltage at said output terminal that is a function of the difference between said floating gate voltage and said input set voltage; said feedback loop for causing the voltage at said first tunnel electrode to be modified as a function of the output voltage, until said differential floating gate circuit reaches a steady state condition such that said floating gate voltage is approximately equal to the input set voltage. According to the present invention, the differential floating gate circuit can then be configured in a read mode to preferably operate as a voltage comparator with a built-in voltage reference or can be configured as a voltage reference circuit for generating an output reference voltage that is a function of the input set voltage.
The present invention is also a method for programming a differential floating gate circuit. According to the present invention, in a floating gate circuit including a floating gate having a voltage thereon, a first tunnel device formed between said floating gate and a first tunnel electrode, and a second tunnel device formed between said floating gate and a second tunnel electrode for causing electrons to tunnel onto and off of said floating gate for modifying the voltage on said floating gate as a function of a voltage differential between said first and second tunnel electrodes, a method of setting the voltage on said floating gate comprises the steps of causing the voltage on said floating gate to be compared to a first voltage, and causing the voltage on said floating gate to be modified using a feedback circuit until said floating gate circuit reaches a steady state condition such that the voltage on said floating gate is a predetermined function of said first voltage.
Alternatively, the method according to the present invention for programming a floating gate in a differential floating gate circuit to an input set voltage comprises the steps of: a) causing a first and second tunnel device coupled to said floating gate to operate in a dual conduction mode under the control of a voltage differential between a first tunnel electrode coupled to said first tunnel device and a second tunnel electrode coupled to said second tunnel device, for modifying the charge level on said floating gate; b) comparing the voltage on said floating gate with an input set voltage at an input voltage terminal and generating an output voltage at an output voltage terminal that is a function of the difference between said floating gate voltage and said input set voltage; and c) causing the voltage at said first tunnel electrode to be modified as a function of the output voltage via a feedback loop coupled between said output voltage and said first tunnel electrode and repeating steps (a) and (b) if said differential floating gate circuit has not reached a steady state condition such that said floating gate voltage is approximately equal to said input set voltage. According to the present invention, the method for programming may also include the step of causing the voltage at said first tunnel electrode and the voltage at said second tunnel electrode to ramp toward a predetermined voltage such that said first and second tunnel devices are no longer in dual conduction. Preferably, the voltages.at said first and second tunnel electrodes are ramped toward ground at the same rate.
An object of the present invention is to provide a method and circuit for generating a voltage reference that has an improved accuracy and stability over the prior art voltage references.
A key advantage of the present invention is the improved initial setting accuracy over prior art floating gate voltage references by more than a factor of 100.
Another key advantage of the present invention is that, without the need for using laser trimming or E2 digital trimming, the present invention has an improved accuracy over bandgap voltage references of a factor of 10 to 50 while drawing less power by a factor of more than 10. Moreover, a voltage reference of greater than or less than 1.2 volts can be set using the present invention without the need for additional amplifiers.
Another advantage of the present invention is that, after a high voltage set mode, the invention allows for a controlled ramp down sequence to ramp down the voltages at the floating gate erase and program electrode such that, when voltage and current sources are completely shut down in the circuit a more accurate voltage is set on the floating gate.
Another advantage of the present invention is that the voltage at the erase electrode is controlled during the ramp down sequence by shutting off the negative charge pump while allowing the feedback circuit to remain active.
Another advantage of the present invention is that tunnel current is used to self-discharge the voltage at each program electrode.
Another advantage of the present invention is the use of a capacitor to control the ramp down rate after shutting off the negative charge pump.
Another advantage of the present invention is that feedback response time tracks tunnel current through the erase and program tunnel devices such that the ramp down rates of the voltages at each erase electrode and program electrode are essentially the same.
Another advantage of the present invention is that the feedback response time is kept slower than the ramp rate so the voltage at the erase electrode never goes below the voltage on the second floating gate in order to avoid oscillation.
Another advantage of the present invention is that the accuracy of setting the floating gate improves as the ramp down time increases.